<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var fullName = 'language';
        var obj = {
            fullName: 'js',
            prop: {
                getFullName: function () {
                    return this.fullName
                }
            }
        }
        console.log(obj.prop.getFullName()) //undefinde
        var test = obj.prop.getFullName;
        console.log(test()) // 'language'
    </script>
</body>

</html>